The present invention relates to an ignition system for internal combustion engines of the type in which the ignition timing is automatically retarded upon occurrence of knocking in the engine, and more particularly this invention relates to such ignition system in which knocking of the engine is detected by the presence of a high frequency pressure variation in the cylinder pressure, whereby the ignition timing is retarded in relation to MBT only when knocking is detected, and in other circumstances the ignition timing is always controlled at around MBT.
The ignition timing of an internal combustion engine must be determined in accordance with the conditions of the engine to ensure optimum operation of the engine. In the past, ignition timing control systems have been generally designed so that the engine speed and intake manifold vacuum are respectively detected by the centrifugal advance mechanism and vacuum advance mechanism so as to represent the engine conditions and thereby determine the ignition timing. And it has been generally believed that in consideration of the engine efficiency and fuel consumption, the best results can be obtained if ignition occurs at around a so-called minimum advance for best torque or MBT, and consequently the ignition timing must be changed to MBT depending on the engine conditions.
Under certain engine conditions, however, advancing the ignition timing around the MBT causes knocking and thus preventing stable operation of the engine. FIGS. 1(A) and 1(B) show the relation between MBT and the knocking limit, and the FIGS. show that the knocking limit is reached before MBT at low speed and low load operations. Also the knocking limit tends to be affected by the atmospheric conditions, e.g., temperature and humidity, and the existing ignition timing control systems are programmed to considerably retard the ignition timing in relation to MBT throughout the range of the engine operating conditions in accordance with the engine speed and intake manifold vacuum parameters. As a result, the power output and fuel consumption are held lower than the performance of the engine. Thus, it is desirable that the ignition timing is controlled at around MBT as long as the engine is not knocking and that the ignition timing is then retarded around MBT only when knocking is caused.
It is generally known that there exists a close correlation between the ignition timing and the cylinder pressure, and the experiments conducted under various conditions on the ratio between the maximum value (hereinafter referred to as a Pmax) of the cylinder pressure obtained when the mixture was exploded and the maximum pressure value (hereinafter referred to as a Pm) obtained under the motoring conditions in which the engine was operated by the motor without exploding the mixture in the cylinders, i.e., the ratio Pmax/Pm, showed that the ratio Pmax/Pm tended to increase as the ignition timing was advanced, and that when the ignition timing was maintained at MBT, the ratio Pmax/Pm was held practically the same irrespective of the parameters, such as, the engine speed, intake manifold pressure, etc., as shown in FIG. 6. (Though not shown in the Figure, the ratio was held practically constant irrespective of the engine cooling water temperature, oil temperature, etc.) It will thus be seen that if the ignition timing of the engine is controlled so as to maintain the ratio Pmax/Pm constant, the ignition timing will be controlled at around MBT. In this case, since it is impossible to detect Pm for the actual control, the ratio Pmax/Pm is detected by detecting the pressure Pi obtained at a fixed angle Ka before the maximum advance angle in FIG. 7 to represent Pm, whereby when the ratio is smaller than a predetermined value, the ignition timing is advanced, whereas when the ratio is greater than the predetermined value, the ignition timing is retarded, thereby maintaining the ignition timing at around MBT independently of the operating conditions of the engine.